A variety of measurement techniques have been employed to measure the wall shear stress generated by fluid flow through pipe. One such method is the pressure gradient technique which relies on the measurement of pressure drop resulting from the flow of a fluid of known rheological properties through a piece of process equipment with known geometry and surface finish. The wall shear stress within the process equipment is related to the process geometry and solution properties through the Navier-Stokes equation. This method is typically used for tubular geometries where the governing relationships are well understood.
Other indirect methods of measurement of wall shear stress include the Preston Tube and Stanton Tube techniques. These techniques utilize specially designed probes with tiny openings facing into the flow, adjacent to the wall. The impact pressure measured in these probes is characteristic of the velocity profile adjacent to the wall and, therefore, also of the shear stress.
Another indirect method for measuring wall shear stress is the Hot Film Anemometer technique which measures fluid shear stress by measuring the changes in heat transfer from a small, electrically heated sensor exposed to the fluid motion. Heat transfer from the probe is governed by the thermal boundary layer which depends on the solution properties and velocity profile in the tube.
Still another method of indirectly measuring wall shear stress is the Diffusion Controlled Electrolysis technique. This technique measures the local wall shear stress from a mass transfer coefficient measured on a small surface element. The mass transfer is determined electrolytically. The small surface element forms one electrode and a much larger (non-controlling) electrode is placed elsewhere in the system. As the voltage between the electrodes is increased, the ion concentration at the cathode surface falls to zero. The current is then governed by the diffusion of the ions through the boundary layer adjacent to the cathode. The mass transfer is then correlated to the wall shear stress.
There is a method of direct measurement of wall shear stress known as the Floating Element technique. This technique utilizes a "floating" tubular section which is exposed to the fluid flow at the interior surface thereof with the floating tubular section being attached to an outer shell with strain measurement devices. The floating section is moved by the wall shear stress exerted by the fluid and the deflection of the floating element is measured. Multiple techniques have been identified for the measurement of the floating element deflection, including piezoelectric crystals and proximity sensitive detectors.
Another method of direct measurement of wall shear stress is the Micromachined Floating Element technique. This technique uses the same measurement concept as that of the Floating Element technique. However, the "floating" section being moved by the fluid flow in the micromachined floating element technique is small as compared to that of the floating element technique.
U.S. Pat. No. 4,464,928 to Dealy discloses an apparatus for measuring wall shear stress where the force that a fluid exerts on a plate, the surface of which is coplanar with the wall of the apparatus, is measured. Several techniques are discussed as possible methods for measuring of the deflection of the plate and, therefore, the force on the plate, including piezoelectric crystals and proximity sensitive detectors. This method is similar to the Micromachined Floating Element technique discussed above.
U.S. Pat. No. 4,790,187 to Tsinober et al discloses a method for measuring wall shear stress where several electrodes are extended into the fluid flow. A magnetic field is established between the electrodes and the shear stress is determined by measuring the relative movement of the electrodes which result in changes in the magnetic field.
U.S. Pat. No. 5,199,298 to Ng et al discloses a method for measuring wall shear stress where a sensor employs a silicon plate suspended about 1.4 microns above the surface of a silicon substrate by means of piezoresistive arms. Deflection of the piezoresistive arms results in a signal which can be converted into a shear stress measurement.
U.S. Pat. No. 4,854,174 to Keith discloses a method for measuring wall shear stress where a hot film shear stress gauge and a piezoelectric pressure transducer are coaxially located along the longitudinal centerline of a cylindrical metal shell. The hot shear stress gauge is exposed to the moving fluid while the pressure transducer is positioned directly below. The conductors attached to the hot film element provide power thereto while the conductors attached to the piezoelectric transducer transmit pressure produced electrical signals therefrom. This method is similar to the Hot Film Anemometer technique discussed above.
U.S. Pat. No. 4,879,899 to Leehey teaches a method for measuring walls shear stress where an elongate body having a longitudinal axis oriented transverse to fluid flow is disposed within the viscous sublayer of the turbulent boundary layer flow across a wall. The body has a plane of symmetry passing through the longitudinal axis of the body and normal to the wall. Torsional springs support the body for deflections about the longitudinal axis and deflections are measured. The angular deflections about the longitudinal axis are substantially linearly related to shear stress on the wall
U.S. Pat. No. 5,052,228 to Haritonidis teaches a method which utilizes a micromachined diaphragm positioned across a gap from an end of an optic fiber. The optic fiber and the diaphragm are integrally mounted. The end of the optic fiber provides a local reference plane which splits light carried through the fiber toward the diaphragm. The light is split into a transmitted part which is subsequently reflected from the diaphragm, and a locally reflected part which interferes with the subsequently diaphragm reflected part. The interference pattern provides an indication of the magnitude and direction of diaphragm deflection. A second fiber optic provides an interference pattern that is out of phase with the first fiber. An interferometer sensor is used as a shear stress measuring device.
In an article entitled "The Effect of Reynolds Number and Mean Velocity of a Flow on Cleaning-In in Place of Pipelines" by D. Timperley, Timperley examined the relationship between microorganism removal and two cleaning solution flow parameters. Those parameters are Reynolds number and mean flow velocity. Timperley examined several flow rates for two different pipe diameters and concluded that microorganism removal correlated with mean flow velocity of the cleaning flow. Timperley then performed a theoretical analysis examining the correlation between the three parameters known to influence the cleaning action of turbulent flow, those being wall shear stress, thickness of the laminar boundary layer, and velocity at the edge of the laminar sublayer, and the measured cleaning solution flow parameters (Reynolds number and mean flow velocity). Timperley's theoretical evaluation showed that the shear stress, as calculated by a turbulent wall shear stress model, the thickness of the laminar boundary layer, and the velocity at the edge of the laminar sublayer all correlated to the mean flow velocity of the cleaning solution and not to the Reynolds number of the cleaning solution.
The prior art fails to teach a method for measuring wall shear stress within a pipe which uses particles adhered to the interior wall of the pipe by a known force such that shear stress can be determined through observation of the removal of such particles with hydrodynamic flow.